Interpretive Summary: We used a combination of qualitative and quantitative techniques to identify genes whose expression patterns a) differ between stress-tolerant and stress-resistant families of the Pacific oyster (Crassostrea gigas) and b) change in response to exposure to pathogenic bacteria known to contribute to mass mortalities of cultivated oysters (Vibrio tubiashii)
W identified 92 gen es that are qualitatively differentially expressed among families using complementary DNA amplified fragment length polymorphism differential displat . We then cloned and sequenced 14 of these fragments, designed fragment-specific primers for quantitative polymerase chain reaction assays , and quantified their expression patterns.. Most of the differences in transcription patterns between stress-tolerant and stress-sensitive families were evident before bacterial exposure, and genes that responded to bacterial exposure did so in parallel in stress-sensitive and stress-tolerant families. BLAST searches of sequence databases revealed that these fragments represent genes involved in immune response as well as genes related to metabolic processes. Our data support the hypothesis that family-level differences in resistance to stress in Pacific oysters are largely attributable to constitutive differences in gene transcription or “general vigor” that are detectable before and maintained after infection rather than to induced responses at the transcriptome-level.

Technical Abstract:
We used qualitative complementary DNA-Amplified Fragment Length Polymorphism (cDNA-AFLP) differential display analysis and real-time, quantitative PCR (RT-qPCR) to identify genes in the Pacific oyster Crassostrea gigas whose transcription either changes in response to exposure to a pathogenic bacterium (Vibrio tubiashii) or varies between families known to differ in sensitivity to heat stress, before and at 12 and 36 h after bacterial exposure at a temperature of 25°C. These conditions simulate those associated with summer mortality syndrome, a poorly understood cause of massive mortalities in cultured Pacific oysters in North America, Asia, and Europe. Using 32 AFLP primer pairs we identified 92 transcript derived fragments that are qualitatively differentially expressed. We then cloned and sequenced 14 of these fragments, designed fragment-specific primers, and quantified their transcription patterns using RT-qPCR. Most of the differences in transcription patterns between stress-tolerant and stress-sensitive families were evident before bacterial exposure, and genes that responded to bacterial exposure did so in parallel in stress-sensitive and stress-tolerant families. BLAST searches of sequence databases revealed that these fragments represent genes involved in immune response as well as genes related to metabolic processes. Our data support the hypothesis that family-level differences in resistance to stress in Pacific oysters are largely attributable to constitutive differences in gene transcription or “general vigor” that are detectable before and maintained after infection rather than to induced responses at the transcriptome-level.